The present invention relates to the shaping of connecting sleeves, or sockets, having an annular bulge on the end of pipes made of a thermoplastic material.
In this operation, such a pipe is heated to the deformation temperature of the plastic material and is pushed into an annular gap formed between a sleeve mandrel and an outer mold, with a free space for the bulge in a widening and compressing device, whereby the sleeve is widened by the sleeve mandrel. The mandrel is provided with a circumferential constriction in the area below the free bulge space and at the same time the pipe is compressed in the axial direction until the annular gap is filled and the initial wall thickness has been increased.
There are various known methods for shaping connecting sleeves having annular bulges on plastic pipes. In one known method, the sleeve and the annular bulge are shaped, after the heated pipe end has been introduced into the annular gap between the outer mold and the sleeve mandrel, by means of movable segments in the sleeve mandrel. This method has the drawback that, due to the movable segments in the sleeve mandrel, it is rather expensive to practice the method primarily since the preparation time and costs are increased.
In a further known method sleeves with annular bulges are shaped with the use of a smooth non-dismantlable mandrel. Thus, for example, the method disclosed in German Offenlegungsschrift (Laid-Open Application) No. 1,924,557, utilizes a smooth sleeve mandrel which is enclosed by an outer mold having an elastic bulge region. The annular bulge is shaped in this process by pressing a defined quantity of an uncompressible medium through openings in the sleeve mandrel into the area of the bulge. The area of the bulge is here sealed against the incompressible medium so that it cannot escape. This is clearly a high-pressure shaping process which is difficult to handle. The drawback of this process is, in particular, the requirement for a defined quantity of incompressible medium whose volume might be changed after each pressing process by residual quantities which remain in the inlet channels. Blowing out the feed channels to prevent such residual quantities upon completion of a shaping process would lead to the loss of the remainder of the incompressible medium and would thus correspondingly increase costs. The result is sleeves with bulge shapes of different sizes and thus a significant reduction in quality. Moreover, when the shaped sleeve bulge region is removed from the sleeve mandrel a portion of the incompressible medium is inevitably left in the sleeve which in addition to renewed loss of medium brings about the requirement for a subsequent cleaning of the shaped pipe section.
German Offenlegungsschrift No. 2,213,561 discloses a further process for forming a sleeve having an annular bulge on the end of a pipe of a thermoplastic material in which a pipe end which has been heated to the required deformation temperature is pushed onto a smooth sleeve mandrel. During this process the end of the pipe is intially widened to the desired size of the sleeve. In a further process step, a compression ring is moved in the direction opposite to the insertion direction to compress the inserted and widened pipe end to such an extent that the annular gap between the sleeve mandrel and the outer mold is filled completely. For a special reinforcement of the sleeve in the region of the bulge, the sleeve mandrel is provided with a circumferential constriction which extends over the area of the bulge and which is also filled with the plastic material during the compression process.
Upon completion of the compression process, a further ring in the outer mold is moved back in the direction of insertion of the pipe to release the bulge area. Thereafter, compressed air is forced through bores in the sleeve mandrel into the bulge area so that the bulge is formed. This known process requires complicated apparatus and is thus limited in its economical use. Moreover, due to the fact that the bulge has to be freed after the compression process by movement of the ring, it is possible to produce only rectangular bulge shapes with this process. Wedge-shaped, circular or other shapes cannot be realized with this process.
U.S. Pat. No. 3,432,887 discloses a procedure in which first a mandrel is inserted into the preheated pipe end in order to initially widen the sleeve region and at the end of the inward stroke of the mandrel, axial compressive forces are applied to deform the sleeve. During this operation, compressive stresses are induced in the pipe material in the region of the bulge. After completion of this compressive deformation, when substantially no axial pressure is imposed on the pipe, air under pressure is introduced into the space enclosed by the pipe in the region of the bulge to blow the bulge outwardly. This induces tensile stresses in the plastic material which do not nullify the compressive stresses because of the successive nature of the operations.